A campaign finance scandal that scuttled his congressional bid and apparently ended the political career of former state Speaker of the House Christopher G. Donovan has quietly ended.

Donovan released a letter Monday from U.S. Attorney Deirdre M. Daly that said the federal probe is finished.

"The investigation is over," the 60-year-old Donovan said in a statement. "I look forward to continuing my efforts to advocate for a fair and just society."

Donovan, a target of the investigation, was never directly implicated in a scheme by Waterbury-based smoke shop owners to funnel $28,000 to his 2012 congressional campaign in an attempt to kill legislation before the General Assembly.

Eight men were convicted in the election scandal, which included multi-year prison terms for Donovan's former finance director, Robert Braddock Jr. of Meriden, and his former campaign manager, Joshua Nassi of Fairfield.

Donovan, a Meriden Democrat who was the early favorite in the race for the 5th Congressional District seat vacated by now-U.S. Sen. Chris Murphy, lost campaign traction after the scandal investigation yielded arrests in the spring of 2012.

Although a former U.S. attorney's private investigation for the Donovan campaign indicated that he had done nothing wrong, Donovan lost the Democratic primary to Elizabeth Esty, a former one-term Democratic member of the state House of Representatives.

"You have asked this Office to provide you with the status of a federal criminal investigation of your client, Christopher G. Donovan, and the financing of his campaign for election to the United States House of Representatives in 2011 and 2012," Daly wrote in a letter released by a Donovan spokeswoman.

"Please be advised that the investigation is now closed and, based on information and evidence currently available to the Government, this Office will not seek to charge your client with violations of federal criminal law in connection with that investigation."

The case dates to August 2011, when the state asked for a court order to designate tobacco shops with Roll Your Own machines as tobacco manufacturers. Customers used the rented machines to roll loose tobacco into cigarettes, thereby avoiding taxes.

While the tobacco dealers won their court case, they correctly sensed that the General Assembly was poised to change the law in 2012.

The campaign contributions were funneled in the names of others, who were illegally reimbursed by the shop owners. An FBI informant was alerted to the attempt to persuade the General Assembly from raising fees on owners of Roll Your Own machines and the investigation resulted.

While the legislation, proposed by the state Department of Revenue Services, died without a vote in the Senate during the General Assembly's 2012 regular session, it became part of the budget later when lawmakers voted on enabling legislation.